Much of the wireless spectrum is licensed by governments or otherwise allocated to particular services or users. It might thus appear that the allocated parts of the spectrum are unavailable to others as they are already occupied. However, in practice, there is often available capacity because the allocated bands are not always fully used by licensed services.
It has been proposed to make more efficient use of the wireless spectrum by including some intelligence in the communicating devices to enable in-use and unused communication channels to be identified, and then an unoccupied one to be selected accordingly for a communications session. This could be particularly beneficial if applied so as to allow unlicensed users to make use of unoccupied licensed channels, providing this does not interfere with licensed usage, which should have priority. This type of approach is described in the white paper “CORVUS: A Cognitive Radio Approach for Usage of Virtual Unlicensed Spectrum”, Jul. 29, 2004, by Brodersen et al, University of Berkley, which is incorporated herein by reference. The concept of using intelligence in communicating devices is known as “cognitive radio”. One definition of cognitive radio has been given by Joseph Mitola as follows: “The term cognitive radio identifies the point in which wireless personal digital assistants (PDAs) and the related networks are sufficiently computationally intelligent about radio resources and related computer-to-computer communications to: (a) detect user communications needs as a function of use context, and (b) to provide radio resources and wireless services most appropriate to those needs.”
Where a user wishes to have access to a network in a cognitive radio environment, there must be a mechanism for making the user's presence known. One proposed method for user discovery suggests using the infrastructure of an existing underlay network, such as a cellular radio access network, which provides extensive coverage over a geographic region. A user retrieves information via the underlay network to enable it to identify and achieve access to a local node, for example, which enables connection to another network. For example, a centralised database provided by the cellular network provider could be accessed via Internet Protocol (IP) messaging. While this proposal would make good use of existing legacy infrastructure, such a solution is not the most spectrally efficient, and cost effective, long-term solution.
In another method, it is suggested that a globally reserved radio band be made available as a conduit for user discovery protocols. The combination of protocols and the global radio band would enable a new user to be discovered. However, the reservation of radio spectrum for the purpose of user discovery is inherently inefficient. It requires the bandwidth to be globally reserved both spatially and temporally. Also, it would require spectrum regulatory bodies to reach international agreement on reserving a frequency band. Furthermore, it would add an unnecessary requirement on modern architectures to support the universally defined radio band.